Detection of Ethylene (C2H4) on Jupiter and Saturn in Non--Auroral Regions

On November 30 and December 2--4 2000 UT, we observed Jupiter and Saturn with the new high--resolution, mid--infrared, grating spectrograph Texes mounted at the NASA/Infrared Telescope Facility (IRTF). In particular, we recorded spectra in the 948-- to 953--cm^-1 range at a resolution of 0.017 cm^-1 to search for ethylene (C₂H₄), a still undetected compound outside of the tempera\-ture--enhanced auroral regions. The integration time was about 15 min. on Jupiter and 30 min. on Saturn with the ~8--arcsec slit oriented North--South and centered on the disk. Another 30 min. was spent on Saturn with the slit 7" east of the central meridian to enhance stratospheric emissions. Spectra in the 1230--1234 cm^-1 region were also recorded to constrain the stratospheric temperature profile. About ten C₂H₄ lines are clearly detected on Jupiter and Saturn. The stratospheric column density inferred from radiative transfer calculations is about 2--3x10¹⁵ molecules cm^-2 on both planets. Recent photochemical models (Moses et al./ 2000, Icarus 143, 244--298 for Saturn; Moses et al./ 2001, Jupiter: Planets, Satellites & Magnetosphere conference proceedings, p. 78, June 25-30, 2001, Boulder, CO for Jupiter) overestimate the actual C₂H₄ abundance on both planets by a factor 3--5. Implications of these results on the hydrocarbon photochemistry will be discussed. One interesting puzzle is the similarity of the C₂H₄ abundances on both planets. In contrast, the C₂H₂ abundance on Jupiter is observed to be a factor of ~2--4 times less that on Saturn. Because the photochemistries of C₂H₂ and C₂H₄ are highly interconnected, these new C₂H₄ observations may provide hints as to the different photochemical or transport processes occurring on the two planets.